Leveling effect of sol–gel SiO2 coatings onto metallic foil substrates

The leveling capability of sol–gel SiO2 layers onto titanium and Kovar (Fe54 Ni29 Co17) foil substrates has been analyzed by atomic force microscopy (AFM) and profilometer roughness measurements as a function of the sol–gel preparation parameters. SiO2 coatings have been prepared by immersion of metallic foils in a solution where the [alkoxide]/[EtOH] ratio was between 0.05 to 0.7, and subsequent withdrawal of the samples at a constant rate between 8 and 49 cm/min. By increasing the [alkoxide]/[EtOH] ratio and/or the withdrawal velocity, the SiO2 layer thickness and leveling capability increase but its mechanical integrity decreases. By increasing SiO2 film thickness, better coverage of large-scale heterogeneities but poorer coverage of short-scale features have been observed. A compact (cracks and striations free) coverage which minimizes the roughness sample surface at short and large scales has been obtained by applying successive SiO2 layers with various thicknesses.